As a mechanism conventionally known for driving a vehicle's power window, there is a motor equipped with a reducer that includes a worm and a worm wheel (see Patent Document 1). In such a reducer, a worm and a worm wheel are housed in a gear housing. The gear housing is expected not only to meet the requirement for lighter weight and smaller size, but also to ensure required strength and dimensional accuracy. For example, there is known a structure in which a rib or ribs is/are formed on the outer surface of a wheel enclosure housing a worm wheel in order to ensure the strength of the gear housing (see Patent Document 2).